Optical disc drives are known, which apply a laser beam to optical discs, i.e., disc-shaped optical recording media, thereby to record data signals on the optical discs or reproduce data signals from the optical discs. Some of the optical disc drives incorporate one optical pickup that can record or reproduce data signals on or from optical discs that differ in specification such as thickness, recording density and the like.
FIG. 1 shows an optical pickup configured to record data signals on optical discs of different specifications or to reproduce data signals recorded on optical discs. As FIG. 1 depicts, an optical pickup 101 has two semiconductor lasers 102 and 103, two beam splitters 104 and 105, a collimator lens 106, an objective lens 107, a lens-supporting mechanism h, a light-receiving element 109, and the like. The semiconductor lasers 102 and 103 are laser beam emitting elements that emit laser beams different in wavelength. The lens-supporting mechanism h includes an actuator that drives and displaces the objective lens 107. The light-receiving element 109 receives the light beam reflected from an optical disc 110.
Various methods are available of enhancing the density at which data signals on optical discs are recorded. Of these methods, the most generally used is to use an objective lens having a large numerical aperture (NA) and apply a light beam having a short wavelength (λ) to read data signals recorded on the optical disc. If the recording medium used is a compact disc (CD), an optical pickup is used that comprises an objective lens having a numerical aperture (NA) of 0.45 and emits a light beam having a wavelength (λ) of 780 nm. If the recording medium is a digital versatile disc (DVD), there is used an optical pickup that comprises an objective lens having a numerical aperture (NA) of 0.6 and emits a light beam having a wavelength (λ) of 650 nm. Note that the diameter of a beam spot formed on the signal-recording plane of the optical disc is proportional to λ/NA. Hence, the beam spot formed on any DVD has a diameter that is about 63% of the diameter of the beam spot formed on the CD.
The larger the numerical aperture (NA) of the objective lens, the more greatly the beam spot will be deformed when the optical disc is inclined to the light beam. More specifically, the beam spot will be more deformed as the coma-aberration increases in proportion to the cube of NA. Thus, the protective layer of the DVD is made thinner than that of the CD so as to reduce the beam-spot deforming caused by the inclination of the optical disc to the light beam.
In order to record or reproduce data signals on or from two types of optical discs, e.g., a CD and a DVD, by one objective lens, the light beam must be focused to form a beam spot of such a size as would record or reproduce data signals as is desired. Such an objective lens is described in, for example, the specification of Japanese Patent No. 2559006.
The CD and the DVD are almost identical in terms of diameter and thickness. However, the protective layer of the DVD is thinner than that of the CD, as pointed out above. The CD and the DVD inevitably differ in the position of the signal-recording plane. Due to this difference, the CD and the DVD are regarded as a thick disc and a thin disc, respectively.
Being thin, optical discs may warp in their radial direction, from the center to the outer circumference. Consequently, so-called “plane wobbling” is likely to occur as any optical disc is rotated. The warping and/or plane wobbling of the optical disc results in a coma-aberration. The coma-aberration gives rise to crosstalk, because the beam spot formed on the disc is deformed. The crosstalk renders unclear signals read from the disc. To minimize the deforming of the beam spot, various methods can be employed. One method is to detect the inclination of the optical disc electrically and to reduce the aberration by an aberration-correcting element in accordance with the disc inclination detected, as is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 11-110802. Another method is to use a triaxial actuator that can move the objective lens in three directions, i.e., focusing direction, tracking direction and tilting direction, as is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2000-36125. These methods described in the Laid-Open publications, i.e., methods of minimizing the deforming of the beam spot formed on the optical disc, require additional components. The additional components render the optical pickup complicated in structure and raise the manufacturing cost of the optical pickup. Ultimately, they will increase the structural complexity and manufacturing cost of the optical disc drive that uses this optical pickup.
A method of preventing the deforming of a beam spot has been proposed. In the method, a two-axis actuator is inclined to the radial direction of the optical disc, in proportion to the displacement of the objective lens from a prescribed position. That is, the actuator is moved to follow the warping of the optical disc, thereby to prevent the deforming of the beam spot. To record or reproduce data signals on or from two types of optical discs that are different in thickness, by means of a single objective lens, it is necessary to adjust the working distance, i.e., the distance between the objective lens and the incidence side of the optical disc. When the same support supports either type of an optical disc and the inclination of the objective lens is set for one type of an optical disc, the objective lens is inclined to the other type of an optical lens by the angle proportional to the difference between the two types of discs in terms of working distance. Inevitably, the optical pickup reproduces degraded data signals.